Optimized glass photographic mask

ABSTRACT

A PROCESS FOR MAKING A PHOTOGRAPHIC OPTICAL GLASS MASK THAT IS USED IN SILICON INTEGRAED CIRCUIT WAFER PROCESSING. AN OPTICAL GLASS MASK IS MADE BY MEANS OF A STEP AND REPEAT CAMERA. DISPLACEMENT ERROR WHICH IS CAUSED BY THE DIFFERENCE IN CEFFICIENTS OF EXPANSION BETWEEN THE MICROSET SCALE OF THE STEP AND REPEAT CAMERA AND THE PHOTOGRAPHIC OPTICAL MASK, IS MINIMIZED BY USING THE SAME MATERIAL FOR BOTH ELEMENTS. FURTHER DISPLACEMENT ERROR CAUSED BY THERMAL MISMATCH OF THE OPTICAL GLASS MASK AND THE SILICON WAFER IS MINIMIZED BY USING A BOROSILICATE GLASS MASK HAVING A LINEAR COEFFICIENT OF EXPANSION OF 3.5X10-6/DEGREES C., WHICH SUBSTANTIALLY MATCHES HE LINEAR COEFFICIENT OF EXPANSION OF THE SILICON WAFER MATERIAL.

3,729,316 OPTIMIZED GLASS PHOTOGRAPHIC MASK Paul P. Castrucci,Poughkeepsie, Nadim F. Haddad,

Hopewell Junction, and Raymond P. Pecoraro, Poughkeepsie, N.Y.,assignors to International Business Machines Corporation, Armonk, N. NoDrawing. Filed Feb. 17, 1970, Ser. No. 12,150 Int. Cl. G03c 5/00 US. Cl.9636.2 2 Claims ABSTRACT OF THE DISCLOSURE A process for making aphotographic optical glass mask that is used in silicon integratedcircuit wafer processing. An optical glass mask is made by means of astep and repeat camera. Displacement error which is caused by thedifference in coefiicients of expansion between the microset scale ofthe step and repeat camera and the photographic optical mask, isminimized by using the same material for both elements. Furtherdisplacement error caused by thermal mismatch of the optical glass maskand the silicon wafer is minimized by using a borosilicate glass maskhaving a linear coefiicient of expansion of 3.5 x IO- /degrees C., whichsubstantially matches the linear coefiicient of expansion of the siliconwafer material.

BACKGROUND OF THE INVENTION The present invention relates to integratedcircuit manufacture, and more particularly to a method of optical maskfabrication which minimized the errors due to thermal effects.

In the manufacture of semiconductor devices, it has been found that bymeans of photolithography, it is possible to achieve fabrication oflarge numbers of units simultaneously in an integrated circuit form. Awafer which comprises the integrated circuit is typically in thevicinity of 2% inch diameter which may represent a'batch of identicalproducts numbering from 100 to several thousand units. To achieve thiscompactness, it is necessary to perform various fabrication processes inminute selected areas over the entire wafer while the balance of thearea is virtually unaffected. Selection of these various areas isusually controlled by means of a mask. Thus, it

is necessary to have a series of masks in order to implement thecomplete processing of the wafer.

In the process of making a photographic glass mask which may be used inthe wafer manufacturing process, it is necessary to make a work platemask from a master photomask. That is, the master photomask is used tomake numerous copies of the master for production use. These copies aregenerally known as submasters. Then, by a further contact printing step,work plates are produced which will be used to expose the photoresistmaterial on the silicon wafer. For the purpose of this specification theterms photomask and photographic mask are interchangeable and refer toeither the master, submaster or workplate referred to above.

In the present state of the art, these photomasks may consist of adeveloped photographic emulsion or thin opaque metallic films depositedon optically flat soda lime glass to selectively expose a photoresistsubstrate.

United States Patent 0 It is this mask which is used on a wafersubstrate. For example, a silicon wafer which is coated with aphotoresist is aligned with a pattern mask and then light is passedthrough the light and dark areas of the mask in order to expose thephotoresist on the silicon wafer. Then, by further processing, thephotoresist is developed so as to enable the removal of undesiredphotoresist film so as to result in a circuit pattern.

Methods of making photographic masks are generally well known in theart. For example several methods are disclosed in the following texts:Integrated Circuits EngineeringBasic Technology, by the Staff ofIntegrated Circuit Engineering Corp., Boston Technical Publishers, 1966;M. Fogiel, Microelectronics-Principles-Design Techniques-FabricationProcesses, Research and Education Association, 1968. One of the morecommon methods is the step and repeat technique, which makes use of astandard step and repeat camera such as the type manufactured by theDavid W. Mann Company. This technique of photographic mask manufactureproduces a two dimensional array of images by a multiplicity.

Each exposure forms a single image in the array. The exposure may bemade by either the contact method or the projection method. If theprojection method is used, then the single integrated circuit patternimage is usually photographically reduced onto the mask plate beingexposed during the step and repeat process. After every exposure, theexposed mask plate is shifted by moving a stepping table on which theplate rests in an XY coordinate system. Movement of the stepping tablemay be controlled by either program or mechanical means.

Commercially available steps and repeat cameras employ a countercontroller that programs and controls the exposure and spacing. Also, amicroset scale is used as a further control of the linear motion of thespacing of the camera.

In general, prior art silicon wafer manufacturing processes did notencounter serious problems due to changes in the temperature of theenvironment in either the mask manufacturing process or in the exposureof the silicon wafer by the mask. This is so because the usuallyexperienced tolerance of .1 mil across a Z A-inch wafer, within atemperature variation of :3 C. was considered to be acceptable withrespect to the size of the circuits present on the wafer. However, withthe advent of greater compactness and an increasing need for furthermicrominiaturization of the circuit areas on the wafer, it has beennecessary to achieve more precise tolerances. Up to the present state ofthe art, it is only possible to control these tolerances by means ofelfecting the environmental temperature changes so as to limit theexpansion of the soda lime glass materials which are used as the supportsurface for the opaque circuit pattern of the mask.

It is therefore a primary object of the present invention to provide animproved photomask to be used in photolithographic silicon waferprocessing.

Another object of the present invention is to reduce the adverse effectof environmental temperature changes in the making of photomasks thatare used in silicon wafer manufacturing processes.

A further object of the present invention is to reduce displacementerrors caused by the differential thermal expansion between thephotographic glass masks and the silicon wafer substrate.

It is a further object of the present invention to match the linearcoefiicient of expansion of the microset scale in a step and repeatcamera with the linear coeflicient of expansion of the photomaskmaterial and further to match this coefiicient of expansion with that ofthe silicon wafer material so as to minimize image displacement errorsdue to differentials in thermal expansion of these materials.

It is a further object of the present invention to use a photomask madeof borosilicate glass having a linear coeflicient of expansionsubstantially similar to the linear coefiicient of expansion of thesilicon wafer material on which the mask will be exposed.

SUMMARY OF THE INVENTION The present mask fabrication techniques exhibita certain amount of mismatch in the laying out of the array by means ofthe step and repeat camera due to thermal effects on the microset scale.Furthermore, there is a tolerance error due to the effect of temperaturedifferences between the silicon wafer material and the mask material.

The microset scale of the step and repeat camera, causes thermal erroras a result of the coefficient of expansion of the microset scale beingdifferent than the linear coefficient of expansion of the mask material.Therefore, the error in the size of the mask is compensated for by theexpansion of the scale by making the scale and the mask material havethe same linear coeflicient of expansion. This may be achieved by usingthe same material for both.

Furthermore, in considering the temperature differences between thesilicon substrate and the mask, image errors due to this differentialare reduced by using a mask material having a substantially similarcoefiicient of expansion to silicon. The material which is used tosatisfy this criteria is borosilicate glass, generally known as Pyrex(registered trademark of Corning Glass Works), which has a thermalcoeflicient of expansion of 3.5x C. Furthermore, both the microset scaleand the mask material are comprised of borosilicate glass therebysubstantially reducing thermal image displacement error.

THEORETICAL ANALYSIS In the generation of a master mask using a step andrepeat camera, the effect of expansion in the original single segmentslide containing the single integrated circuit pattern image isnegligible, since it takes place over a chip length rather than over alarge water diameter.

If the cameras microset scale is made of the same material as the masks,the error in the size of the master mask is compensated by the expansionof the scale; if not, an error resulting from the different expansion ofscale and master will result in generating masters with different sizes.This effect is analyzed to determine maximum error in the followingmanner.

Let the temperature in the mask fabrication area be held within :At C.)of t Furthermore, assume that the temperature of the camera scale andplates follow room temperature. If a master of intended size d is shotat tg+At;, its size at the reference temperature t is offset by :6; (theerror introduced due to difference of expansion between microset scaleand master mask plate), depending upon whether the plate expands lessthan the scale or vice-versa; then,

where C and C are the coeflicients of thermal expansion of the camerascale and mask, respectively. If both the scale and plate are made ofthe same material, 6;:0 and all masters are the same size.

Similarly, if the master is shot at t,-At;, the 2; size would be offsetby 16 The maximum variation in the size of any two masters at the sametemperature, therefore is 26:.

The smallest size submaster would be generated from the smallest sizemaster at t At The pattern size at that temperature would be d-6 dc AtThe extra term being due to cooling the master by Ai At the referencetemperature of I the smallest submaster would then have an expandedpattern size of d 6;, the same as the smallest size master.

The maximum size pattern on any submaster would be generated through theuse of the maximum size master (d+6;) at the maximum temperaturepossible (ti-FAQ). The pattern size would then "be d+6;+dC At whichreduces to (1+6; at t, temperature.

The same argument holds for the generation of work plates from the submasteus. The maximum variation in the pattern size on any two workplates at any one temperature, therefore, is 26,.

Now assuming that the temperature in the wafer exposure area can varyfrom 23,-431 to t+Ar the incremental expansion in the pattern size onany mask due to Ar is then 6 =dC At The corresponding expansion of anypattern of dimension d on the wafer would be governed by the expansionof silicon. Let 6 =dC Ar Where C is the thermal expansion coeflicient ofthe silicon wafer at room temperature. Then, if C C it could be seenthat a maximum mismatch, I between any two consecutive patterns occurswhen the smallest size mask (ti-6,) is exposed at the lowest temperature(r -At while the next pattern uses the maximum size mask (d+ 6f) atmaximum temperature (r +At The size of the smallest mask pattern at t-At is d -6 -6 This pattern is transferred to the silicon wafer. Whenthe next pattern is exposed at t +At the size of the initial patternbecomes d6,--6 -+26 The additional 26 term is the expansion of thesilicon wafer due to 2At increase in temperature. At that temperature,the maximum size mask pattern becomes d+5f+6 Therefore,

E= (d+ 5.5+ 6 (a'6 6 ew) r+ em ew) where C C Then, for the generalcondition:

The second term being due to the difference in the expansion coefficientbetween mask and wafer. If these coefficients were the same, the secondterm would vanish and the maximum mismatch would be the same as that onthe mask pattern. Therefore, to minimize the mismatch between patterns,the linear coeflicient of thermal expansion of the mask material as wellas the scale on the step and repeat camera should be close as possibleto that of silicon.

APPLICATION The maximum mismatch, E, could be rewritten as E=E+E whereEf 2dAtf|Cc Cml =Mask fabrication area contribution Il=2dAt |C -C=Exposure area contribution.

At present, soda-lime glass is used in photographic mask fabrication.The thermal expansion coefficient of this material is three and a halftimes as great as that of silicon. This results in a relatively largemismatch between patterns. To minimize this mismatch, it is necessary touse a material with a linear coefficient of expansion substantiallysimilar to that of silicon. This is achieved by using borosilicateglass, generally known as Pyrex, which has a coefiicient of expansion of3.5 X 10-/ C.

Table I lists the values of E, for different temperature controltolerances in the mask fabrication area, while Table II lists the valuesof E, for different temperature control tolerances in the exposure(photo-resist) area.

The following parameters were used in deriving the TABLE IVPMAXIMUMMISMATCH, (MILQ'DUE To values in Tables I and H. PERATURE DIFFERENCEBETWEEN WAFER AND MASK v I I d=d1ameter of Wafer=2 A 1n. Thermalexpansion coefiicient of soda-lime glass Mask material -=9.2 C. 5Sodalime glass 0.021 0. 041 0.002 0.083 0.104 Thermal expansioncoefficient of Pyrex glass Pyrex glass 1016 (1024 M32 M39 C- While theinvention has been particularly shown and Thermal expansion coeflicientof si1icon=2.6 10- C. described With reference to the preferredembodiment TABLE L-MASK FABRICATION AREA CONTRIBUTION To MISMATCH, Er(MILS), VERSUS RooM TEMPERATURE CONTROL Stop and repeat camera scaleMask material =l=1 5:2 5:3 i4 :l:5

Sodalime glass Sodalime glass 0. 000 0.000 0.000 0.000 0.000 Pyrex glass0. 020 0. 051 0. 077 0.103 0.128

Borosllicate glass Sodalime glass 0.026 0.051 0.077 0.103 0.128Bor0silicateg1ass 0.000 0.000 0.000 0.000 0.000

TABLE H'IEXPOSURE AREA CONTRIBUTION TO thereof, it will be understood bythose skilled in the art P 4338 EB (MILSLVERSUS ROOM TEMPERATURE thatvarious changes in form and detail may be made therein without departingfrom the spirit and scope of 0 s the lnvention. Mask material 11 i2 :l:35:4 =l=5 What I claim is: sodahme glass 0,030 0,059 0.089 0.119 0,149 InProcess for makmg a Photohthograhhlc mask Borosilicate glass 0.004 0.0080.012 0.010 0.020 for use 1n the manufacture of 51116011 waferintegrated circuits, wherein the photographic emulsion on said mask Itis clear from the data in the tables that the best is exposed with aplurality of images of the integrated result is achieved if both thescale and masks are made circuit by means of a step and repeat cameraand the 0f Pyrex glass, because the thermal eXPaIIStOH coeificientphotographic emulsion on the silicon wafer is exposed of Pyrex glass isthe closest to that of silicon. with a plurality of images of theintegrated circuit by Now, in considering the temperature difierencesbe- 0 means of contact printing through said mask, the imtween themicroset scale of the step and repeat camera movement comprising: andthe master mask plate and between the Work plate supporting thephotographic emulsion on said mask and the water, it will be assumedthat the scale and mask with a plate of borosilicate glass having alinear cor made 0f the Same kind of glass (bofosiheate)- eflicient ofthermal expansion substantially similar Let tc=tempefature of the cameraScale; m= master to the coefiicient of expansion of the silicon wafer,

Plate temperature; then, "r= ml c-- m|= Hits indexing said step andrepeat camera with respect to match. said mask by means of a microsetscale composed of The mismatch "r is also introduced When generating thea borosilicate glass having a linear coefficient of ther- Sllbmastefs ifthere is a temperature difference between mal expansion substantiallysimilar to the coefiicient the submaster and the master. It is alsointroduced when of expansion of the silicon wafer, work plates aregenerated. This effect is additive (or subwhereby the size of theplurality of images of the intetractive). grated circuit to which thesilicon wafer is exposed, I11 the exposure area, and m w the Worst Caseis independent of differences in the temperature at expansion occurswhen the wafer temperature is held conwhich each photographic exposurestep wa stant while the temperature of the mask varies. If t is r d, thewafer temperature and r is the mask temperature, 2. In a process formaking photolithographic masks then: for use in the manufacture ofsilicon wafer integrated circuits, wherein the photographic emulsion ona master n =dC t -r =induced mismatch ex osure area comml m WI p mask isexposed with a plurality of 1mages of the lnteponent).

grated circuit by means of a step and repeat camera, the Then, for thegeneral case: photographic emulsion on a submaster mask is exposedne=greater f c l l or dc l with said plurality of images of theintegrated circuit by contact printing through said master mask, thephoto- By reference to Tables and IV l 11st and graphic emulsion on awork plate mask is exposed with for Severa1.t.emperature dlfierences It18 clear that. the said plurality of images of the integrated circuit byconuse of .bOI'OSIIICalC glass masks produces substantially tact Primingthrough Said submaster mask and the less mismatch than that encounteredwith soda-hme photographic emulsion on the Silicon wafer is exposedmaskswith a plurality of images of the integrated circuit by TABLEIIL-MAXIMUM M MA'IC (M m). DUE TO E contact printing through said workplate mask, the im- PERATURE DIFFERENCE BETWEEN CAMERA SCALE AND MASTERMASK, 0R BETWEEN MASK PLATES EUR pfovemeht compflslng- ING CoNTAoTPRINTING supporting the photographic emulsion on sa1d master PM! o O)mask with a plate of borosilicate glass having a linear coefiicient ofthermal expansion substantially similar Mask material to the coefficientof expansion of the silicon wafer, Sodalime glass 0.021 0. 041 0. 002 0.088 .1 4 indexing said step and repeat camera with respect toBoroslhcate glass 0'008 0'016 0'032 0'039 said master mask means of amicroset c l composed of a borosilicate glass having a linear coefiiisindependent of differences in the temperature at cient of thermalexpansion substantially similar to which each photographic exposure stepwas executed. the coefficient of expansion of the silicon wafer,

supporting the photographic emulsion on said subefere ce Cited mastermask f\gith a plate lot bOIiOSiIiCatC glass having 5 UNITED STATESPATENTS a linear coe cient o t erma expansion su stan- 3,567,447 3/1971Chand 96--38.3 giizliilgnsirwnglil-r, to the coefficient of expansion ofthe 3,355,291 11/1967 Baird et a1 96 38.4

supporting the photographic emulsion on said work plate mask with aplate of borosilicate glass having NORMAN TORCHIN Primary Examiner alinear coefiicient of thermal expansion substan- J. L- GOODROW,Assistant Examiner tially similar to the coefiicient of expansion of thesilicon wafer, US. Cl. X.R.

whereby the size of the plurality of images of the integrated circuit towhich the silicon wafer is exposed 15

